The Apparent Simplicity of RGB Rendering

by kelsolaaron 1/30/2024, 8:41:59 AM with 5 comments
  • by meindnochon 1/31/2024, 12:35:40 PM

    Tristimulus values (X, Y, Z) are the dot product of the incoming spectral radiance (L) and the spectral sensitivity curves (x, y, z) of the three cone cell types:

      X = dot(L, x)
  Y = dot(L, y)
  Z = dot(L, z)
    The spectrum of the light reflected from a surface (Láµ£) is the pointwise product of the incoming spectral radiance (L) and the spectral reflectance curve of the surface (R):

      Láµ£ = L * R
    What RGB rendering does, is it replaces this equation (for x, y, z):

      X = dot(Láµ£, x) = dot(L * R, x) (eq I)
    with this (for x, y, z):

      X = dot(L, x) * dot(R, x) (eq II)
    Which are not equal in the general case. On the other hand, they are equal up to a scaling factor if L is a constant function, since in that case:

      dot(L * R, x) = C * dot(R, x) (from eq I)
  dot(L, x) * dot(R, x) = D * dot(R, x) (from eq II)
    where C and D are scaling factors that depend solely on L. The interpretation of this is that as long as the light source has a sufficiently flat spectrum (like sunlight!), the difference between spectral rendering and RGB rendering will be negligible.

  • by alhirzelon 1/31/2024, 6:37:20 AM

    Those interested in this article may be interested in experimenting with the Mitsuba renderer [1]. This is one of the best open-source spectral renderers, and it includes support for RGB and spectral modes (in addition to propagating things like polarization), all in one unified rendering pipeline with CUDA and non-CUDA backends.

    [1]: https://www.mitsuba-renderer.org/

  • by lsaferiteon 1/31/2024, 1:46:08 PM

    I've been working with a hyperspectral camera (350-1000nm,4nm,12b,410x410,8Hz) lately and this article was quite interesting through that specific lens. I'm pretty much in love with hyperspectral imaging now and it's really opened my eyes to a whole new world of information and technology. Not super relevant, just thought I'd share. Also, this camera has really made me want a real-life tricorder.

  • by zokieron 1/31/2024, 6:48:39 AM

    It is baffling just how well RGB rendering works despite it being such a huge hack; many real-world scenes are practically indistinguishable between rgb and spectral rendering. But still I find the physically based rendering pretty disingenuous considering the huge gap still remaining between real-world and standard pbr although I accept that it is just a domain-specific term that shouldn't be taken too literally.

    Just to spark discussion, here is one example of research aiming to reduce that gap and bring more physicality: https://ssteinberg.xyz/2023/03/27/rtplt/